In order to be able to immerse in deep water without making the vessel excessively heavy, hulls of submarines are generally constituted by sheets of steel having a thickness of between 40 and 50 mm, and optionally forged components which have a thickness of between 100 and 150 mm and which are constituted by a very high strength steel having a very good impact resistance even at low temperatures, so as to provide a good degree of reliability even in the case of intense dynamic loads, and which can be relatively easily welded so as to allow assemblies of a high quality to be produced.
The steels which are conventionally used are steels of the series referred to as 60 or 80 HLES, whose chemical composition comprises approximately 0.10% of carbon, from 2 to 4% of nickel, from 0.2 to 0.4% of silicon, molybdenum and vanadium at contents such that Mo+3V is between 0.3 and 0.5%, between 0.8 and 1.2% of Mn, between 0.1 and 0.5% of Cr, the balance being iron, impurities and optionally low quantities of deoxidation elements. These steels are used to produce components such as sheets or forged components which are quenched and tempered so as to have a tempered structure which is on the whole martensitic, that is to say, which contains more than 90% of martensite and of which the yield strength is between 550 and 650 MPa, the tensile strength is between 600 and 750 MPa, the elongation at break is between 15 and 20%, the Charpy toughness Kcv is greater than 80 J at −80° C.
Components which are produced from these steels are assembled by means of welding with preheating to a temperature in the order of at least 150° C. in order to prevent problems of cracking in the cold state.
These welding conditions are required in particular because the weld seams which are produced are weld seams which are very extensively flanged and which can produce stresses of almost 80% of the elastic limit, and because the weld seams are carried out on sites in which the temperature may drop to a level in the region of 0° C.
The need for carrying out a preheating operation at a high temperature is a disadvantage which makes it difficult to weld components of submarine hulls. Therefore, it is desirable to be able to have a steel which allows weld seams to be produced under less severe conditions, that is to say, without pre-heating, or at least by carrying out only a baking operation of the sheets which does not exceed 100° C., or preferably, 50° C., in spite of the very extensive flanges of the weld seams and in spite of the relatively low outside temperatures of the site.
It has been proposed, in particular in patent application WO93/24269, that the welding conditions be improved for submarine hulls which are produced from a steel of the 60 or 80 HLES type, by using welding electrodes which are different from the electrodes defined by the construction standards which are applied in this field, these welding electrodes leading to low-carbon bainitic structures (LCBS).
However, this technique has disadvantages since the reduction of the risk of cracking which is thus obtained in the region of the deposited metal nonetheless does not overcome the problem of the risk of cracking brought about by the welding operation in the base metal itself, in the region of the heat affected zone.